If the substance doesn't change chemically, it is a physical reaction.
When a force is applied to the box , this will cause an acceleration to the box.
(force =mass×acceleration)
So the box has a constant acceleration and a changing velocity.
To calculate the force between two negative charges, we use the formula which is given by the Coulomb`s Law as

Here,
and
are the charges on the pith balls, r is the separation between the charges and k is constant and its value is
.
Given
and
.
Substituting these values in above formula we get,

Thus, the repulsive force between two pith balls is
.
The astronaut should shoot the aerosol spray in the opposite direction of where the astronaut is going to return back to the spaceship. This is in accordance to Newton’s Thrid Law, every action has an equal and opposite reaction.
Hope this helps you out!